diff --git a/ChangeLog.txt b/ChangeLog.txt
index c6cee2b0..509dbaf8 100644
--- a/ChangeLog.txt
+++ b/ChangeLog.txt
@@ -23,6 +23,8 @@ Broker:
 - Disable TLS renegotiation. Client initiated renegotiation is considered to
   be a potential attack vector against servers. Closes #1257.
 - Fix incorrect shared subscription topic '$shared'.
+- Fix zero length client ids being rejected for MQTT v5 clients with clean
+  start set to true.
 
 Client library:
 - Fix typo causing build error on Windows when building without TLS support.
diff --git a/src/handle_connect.c b/src/handle_connect.c
index 1bd35ace..9de98dd9 100644
--- a/src/handle_connect.c
+++ b/src/handle_connect.c
@@ -540,8 +540,12 @@ int handle__connect(struct mosquitto_db *db, struct mosquitto *context)
 			}else{
 				allow_zero_length_clientid = db->config->security_options.allow_zero_length_clientid;
 			}
-			if(clean_start == 0 || allow_zero_length_clientid == false){
-				send__connack(db, context, 0, CONNACK_REFUSED_IDENTIFIER_REJECTED, NULL);
+			if((context->protocol == mosq_p_mqtt311 && clean_start == 0) || allow_zero_length_clientid == false){
+				if(context->protocol == mosq_p_mqtt311){
+					send__connack(db, context, 0, CONNACK_REFUSED_IDENTIFIER_REJECTED, NULL);
+				}else{
+					send__connack(db, context, 0, MQTT_RC_UNSPECIFIED, NULL);
+				}
 				rc = MOSQ_ERR_PROTOCOL;
 				goto handle_connect_error;
 			}else{
diff --git a/test/broker/12-prop-assigned-client-identifier.py b/test/broker/12-prop-assigned-client-identifier.py
index 376d10ce..28f5037b 100755
--- a/test/broker/12-prop-assigned-client-identifier.py
+++ b/test/broker/12-prop-assigned-client-identifier.py
@@ -5,43 +5,48 @@
 
 from mosq_test_helper import *
 
-rc = 1
 
-keepalive = 10
 
 
-connect_packet = mosq_test.gen_connect(None, proto_ver=5, keepalive=keepalive)
+def do_test(clean_start):
+    rc = 1
+    keepalive = 10
+    connect_packet = mosq_test.gen_connect(None, proto_ver=5, keepalive=keepalive, clean_session=clean_start)
 
-props = mqtt5_props.gen_string_prop(mqtt5_props.PROP_ASSIGNED_CLIENT_IDENTIFIER, "auto-00000000-0000-0000-0000-000000000000")
-connack_packet = mosq_test.gen_connack(rc=0, proto_ver=5, properties=props)
+    props = mqtt5_props.gen_string_prop(mqtt5_props.PROP_ASSIGNED_CLIENT_IDENTIFIER, "auto-00000000-0000-0000-0000-000000000000")
+    connack_packet = mosq_test.gen_connack(rc=0, proto_ver=5, properties=props)
 
-props = mqtt5_props.gen_uint32_prop(mqtt5_props.PROP_SESSION_EXPIRY_INTERVAL, 1)
-disconnect_client_packet = mosq_test.gen_disconnect(proto_ver=5, properties=props)
+    props = mqtt5_props.gen_uint32_prop(mqtt5_props.PROP_SESSION_EXPIRY_INTERVAL, 1)
+    disconnect_client_packet = mosq_test.gen_disconnect(proto_ver=5, properties=props)
 
-disconnect_server_packet = mosq_test.gen_disconnect(proto_ver=5, reason_code=130)
+    disconnect_server_packet = mosq_test.gen_disconnect(proto_ver=5, reason_code=130)
 
-port = mosq_test.get_port()
-broker = mosq_test.start_broker(filename=os.path.basename(__file__), port=port)
+    port = mosq_test.get_port()
+    broker = mosq_test.start_broker(filename=os.path.basename(__file__), port=port)
 
-try:
-    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
-    sock.settimeout(10)
-    sock.connect(("localhost", port))
+    try:
+        sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+        sock.settimeout(10)
+        sock.connect(("localhost", port))
 
-    sock.send(connect_packet)
-    connack_recvd = sock.recv(len(connack_packet))
+        sock.send(connect_packet)
+        connack_recvd = sock.recv(len(connack_packet))
 
-    if connack_recvd[0:12] == connack_packet[0:12]:
-        # FIXME - this test could be tightened up a lot
-        rc = 0
+        if connack_recvd[0:12] == connack_packet[0:12]:
+            # FIXME - this test could be tightened up a lot
+            rc = 0
     
-    sock.close()
-finally:
-    broker.terminate()
-    broker.wait()
-    (stdo, stde) = broker.communicate()
-    if rc:
-        print(stde.decode('utf-8'))
-
-exit(rc)
+        sock.close()
+    finally:
+        broker.terminate()
+        broker.wait()
+        (stdo, stde) = broker.communicate()
+        if rc:
+            print(stde.decode('utf-8'))
+            exit(rc)
+
+
+do_test(True)
+do_test(False)
+exit(0)